The present invention relates to a method of plugging an elongate annulus. The method is particularly, though not exclusively, suited to assist recovery of control of a well for hydrocarbons following a "blow-out", the annulus then being between two tubulars in the well.
A typical well in the North Sea comprises a 30" diameter outer tubular, known as a conductor, with 20", 133/8" and 95/8" inner tubulars known as casings, arranged within each other inside the conductor. Inside the 95/8" tubular, known as the production casing, there may be present during a blow-out either no further tubular, or 7" production tubing or other tubing or a drill string for example. The blocking of a blowing stream in a central tubular has been the subject of prior proposals, for example as described in U.S. Pat. No. 3,647,000, in which crimping of the tubing is disclosed to be followed by the injection up-stream of the crimp of solid particulate material which lodges at the crimp and blocks the tubing. Another proposal has been that a blowing annulus--between two tubulars--be blocked by lodging at a collar steel chain having cast on rubber balls. However, sealing onto an outer tubular, plugging a non-blowing but possibly pressurised annulus within the outer tubular, and reaching on through to block the central blowing tubular without breaking the outer seals, has not been considered previously.
As is described in our earlier British patent application No. 7,926,138--from which our British patent application No. 8,022,921 claims priority and which also describes apparatus for use in underwater blow-out control by effecting side entry--control operations may be started by cutting access windows in the 30" conductor and at least the outer casing with explosive charges.
However, to cut an access window by any method in the 95/8" production casing may well be an unacceptable risk and both the annulus and the production tubing must be plugged and blocked with plugging and blocking material being passed through apparatus sealed onto the casing. For example, when blow-out control operations are started and the 95/8" casing is hot-tapped pressure may be found in the annulus which cannot be bled off. Even if there is no annulus pressure initially, when the blowing production tubing is blocked and pressure builds up behind the block, then pressure comes on to down hole seals which may break down. Further, on blocking of the production tubing it may fail as a result of having been eroded and weakened. Thereafter, in either case, pressure will rise in the annulus, which pressure will not be contained by proven seals and an underwater blow-out will be likely to follow, unless steps disclosed below are taken. Thus if an access window is cut in the production casing to regain control of a platform blow-out or one occurring at the sea surface, if the platform has burnt out, there is a significant risk of worsening the situation by creating an underwater blow-out at the access window. The risk of this outcome may not be acceptable.